Apparatus for crucible-free zone melting



Sept. 19, 1967 R. EMEIS 3,342,970

APPARATUS FOR CRUCIBLE-FREE ZONE MELTING Filed Feb. 4, 1965 a 1. Fig?Fig. 1

United. States Patent Germany Filed Feb. 4, 1965, Ser. No. 430,361Claims priority, appligatigrzGermany, Feb. 8, 1964,

6 Claims. (31. 219-1043 ABSTRACT OF THE DISCLOSURE Apparatus forcrucible-free zone melting of a crystalline rod includes an inductionheating coil for forming a melting zone in the rod. The coil surroundsand is spaced from the rod and is energizable to produce an inductiveheating field around the rod. The coil and rod are relatively movable inthe direction of the rod axis so as to pass the melting zone along therod. The rod extends through an opening of a concentrator device whichis located in the field between the heating coil and the melting zoneand comprises a plurality of parts movable relative to each other withinthe field for varying the size of the opening so as to accommodate rodsof respectively varying cross section therein. The concentrator devicecan be a subdivided ring with a silver surface, the subdivision of thering being radially movable, or can comprise a pair of silvered metalstrips aligned end to end and formed 3,,34Z,9 70 Patented Sept. '19,1967 ticularly of semiconductor material, having two holders grippingthe ends of the body and an induction coil surrounding and spaced fromthe rod for producing the melting zone in the rod, the induction coilbeing movable relative to the longitudinal axis of the rod. Aconcentrator device surrounding the rod is arranged in the field betweenthe heating coil and the melting zone. The concentrator is divided intoa plurality of parts adjustable with respect to each other so as to varythe area encompassed thereby.

Other features which are considered as characteristic for the inventionare set forth in the appended claims.

Although the invention is illustrated and described herein as embodiedin a device for crucible-free zone melting, it is nevertheless notintended to be limited to the details shown, since various modificationsand structural changes may be made therein without departing from thespirit of the invention and within the scope and range of equivalents ofthe claims.

The construction and method of operation of the invention, however,together with additional objects and advantages thereof will be bestunderstood from the following with a V-shaped notch at mutually adjacentends thereof defining an aperture through which the rod extends.

My invention relates to apparatus for crucible-free zone melting of arod-shaped member of crystalline material. Such apparatus is known indiiferent embodiments. Usually, a crystalline rod that is to beprocessed is located in an evacuated vessel or in a vessel filled withprotective gas. The rod consists, for example, of a semiconductormaterial such as germanium orsilicon, or a metal, such as tungsten ormolybdenum. The ends of the rod are gripped by holders and the rod isusually located in a vertical position. A heating device heats the rodalong a narrow length thereof and thus forms a melting zone. Thismelting zone is passed along the length of the rod through relativemovement of the heating device and the rod with respect to thelongitudinal axis of the rod.

In heretofore known apparatus, an induction heating coil which surroundsand is spaced from the rod, serves as a heating device for theproduction of the melting zone. Thus, in the case of silicon, forexample, the heating coil can be energized with a current having afrequency of 1-5 MHz. The semiconductor body can thus be melted byinductive heating produced by the coil. As a rule, the heating coil mustsurround the semiconductor rod relatively closely, in order to providegood coupling. This, of course, limits the growth diameter of the rod.

It is accordingly an object of my invention to provide a device forcrucible-free zone melting which avoids the coupling difficultiesencountered heretofore.

It is an additional object of my invention to provide apparatus forcrucible-free zone melting which will make it possible to processcrystalline rods of various cross sections and with variouscross-sectional dimensions, without having to change the heating coil.

A further object of the invention is to provide a device which would aidalso in producing crystalline rods whose cross-sectional dimensions varyalong the rod length.

With the foregoing objects in view, I provide apparatus forcrucible-free zone melting of a crystalline rod, pardescription ofspecific embodiments when read in connec tion with the accompanyingdrawings, in which:

FIG. 1 is an elevational view of an embodiment of my invention showing aconcentrator and induction heating coil in position around the moltenzone of a crystalline rod.

FIG. 2 is a top plan view of FIG. 1 with the induction heating coilremoved.

FIGS. 3 and 4 areplan and elevational views, respectively, of anadditional embodiment of a concentrator constructed in accordance withmy invention.

Referring now to the drawings, and first, particularly, to FIG. 1, thereis shown a melting zone 4 produced in a rod-shaped body 2, which may,for example, have a circular cross section. The melting zone 4 isproduced by heating with an induction heating coil 3, which may be inthe form of a fiat, helically wound coil. The field of the heat-' ingcoil 3 is concentrated by a concentrator device consisting of the parts5a, 5b, 5c and 5d (FIG. 2). Concentrators per se are known from theinductive heating arts. They constitute divided or split rings ofelectrically conductive material which are located in the field of aninduction coil. Due to the division of the concentrator, only such eddycurrents are produced respectively within the individual parts as wouldultimately concentrate the field in the area of the opening inside theconcentrator. A closed ring, on the other hand, does not constitute aconcentrator, because, with a closed ring, a field opposite to the fieldof the heating coil is produced, and the heating effect is completelynullified.

In the case at hand, where a concentrator is used for crucible-free zonemelting, it can consist, preferably, of silver or of a copper componentprovided with a silver coating and connected preferably to a coolingloop, for example a water cooling system.

The German published patent application No. 1,094,710 discloses a methodby means of which monocrystals are produced byseed crystals 'having adiameter considerably smaller than that of the semiconductor rod beingprocessed. FIG. 1 shows such a crystal seed 6. With the device of myinvention, no difficulties whatsoever are encountered in the processingof such a semiconductor rod since, by adjusting the concentrator, a typeof concentration of the electromagnetic field may be obtained whichmakes the coupling of the heating coil 3 equally satisfactory in allpositions. Thus, for example, the narrow crystal seed may have adiameter of approximately 3 to 5 mm., while the semiconductor rod 2 mayhave a diameter of 12 to 40 mm.

FIG. 2 indicates by the double-headed arrows the directions of motion ofthe parts 5a-5d of the concentrator 5,

which permit adjustment of the concentrator to the varying crosssections of the rod-shaped body to be processed. The device of myinvention is also especially employable in cases where a desired changein the cross section of the rod, i.e. an enlargement or reduction, istobe effected by means of the crucible-free zone melting process.

FIGS. 1 and 2 illustrate a concentrator which has a circular ring-shapedcross section and which is subdivided into four parts. It is, of course,understood that the concentrator may also be divided into two or threeor more than four parts. The shape of the opening in the concentratormay also conform to the cross sectional shape of the rod being treatedwhich can be square or rectangular, for example.

FIGS. 3 and 4 show another embodiment of a concentrator according to theinvention pnd consisting of two strips 11 and 12, aligned end to end andformed with V- shaped notches at the mutually adjacent ends. The strips11, 12 are movable in the directions of the two-headed arrows.

If necessary, the concentrator can be made tolie completely inside thewinding of the heating coil and be movable only from the outside of theheating coil. It is also possible to provide a concentrator above orbelow or both above and below the heating coil.

It is advisable to have all parts of the device of my invention enclosedby a relatively large vessel, which serves for maintaining the necessaryvacuum, or the protective gas atmosphere, respectively as shown withrelated equipment, for example, in my Patent No. 2,972,525. Theconcentrator may also be placed in a relatively narrow vessel, forexample a quartz tube, while the heating coil in located outside of thevessel, for example surrounding the quartz tube.

Relative motion between the heating coil and the treated rod may beeffected by movement of the heating coil while the rod is tightlygripped inside the vessel, or by moving the rod while the heating coilis held fixed.

In the device of my invention, movement of the rod holder while theheating coil and the adjustment concentrator are fixedly mounted,appears to be more advantageous.

Movement of the parts 5a-5c (FIGS. 1 and 2) can be effected by suitablymanipulating the holder rods 7 secured thereto.

Movement of the strips 11, 12 (FIGS. 3 and 4) can be effected bysuitably manipulating the distant ends thereof.

I claim:

1. In apparatus for crucible-free zone melting of a crystalline 'rod, aninduction heating coil for forming a melting zone in the rod, said coilsurrounding and spaced from the rod and being energizable to produce aninductive heating field around the rod, said coil and rod beingrelatively movable in the direction of the rod axis so as to move themelting zone along the rod, and a concentrator device having an openingthrough which the rod extends, said concentrator device being located inthe field between the heating coil and the melting zone and comprising aplurality of parts movable relative to each other within said field forvarying the size of said opening so as to accommodate rods ofrespectively varying cross section therein.

2. In apparatus for crucible-free zone melting of a crystalline rod, aninduction heating coil for forming a melting zone in the rod, said coilsurrounding and spaced from the rod and being energizable to produce aninductive heating field around the rod, said coil and rod beingrelatively movable in the direction of the rod axis so as to move themelting zone along the rod, and a concentrator device having an openingthrough which the rod extends, said concentrator device being locate-dbelow the heating coil in the field between the heating coil and themelting Zone and comprising a plurality of parts movable relative toeach other within said field for varying the size of said opening so asto accommodate rods of respectively varying cross section therein.

3. Apparatus according to claim 1 wherein said plurality of parts isgreater than two.

4. In apparatus for crucible-free zone melting of a crystalline rod, aninduction heating coil for forming a melting zone in the rod, said coilsurrounding and spaced from the rod and being energizable to produce aninductive heating field around the rod, said coil and rod beingrelatively movable in the direction of the rod axis so as to move themelting zone along the rod, and a concentrator having an opening throughwhich the rod extends, said concentrator being located in the fieldbetween the heating coil and the melting zone and comprising asubdivided ring having a silver surface, the subdivisions of said ringbeing radially movable for varying the inner diameter of said ringwhereby rods of varying cross-sectional area or shape can beaccommodated by said concentrator.

5. In apparatus for crucible-free zone melting of a crystalline rod, aninduction heating coil for forming a melting zone in the rod, said coilsurrounding and spaced from the rod and being energizable to produce aninductive heating field around the rod, said coil and rod beingrelatively movable in the direction of the rod axis so as to move themelting zone along the rod, and a concentrator comprising a pair ofmetal strips aligned end to end each formed with a V-shaped notch atmutually adjacent ends thereof defining an aperture through which therod extends, said concentrator being located in the field between theheating coil and the melting zone, and the metal strips of saidconcentrator having a silver surface and being movable toward and awayfrom each other for varying the size of said aperture whereby rods ofvarying cross-sectional area or shape can be accommodated by saidconcentrator.

6. Apparatus according to claim 1 wherein said concentrator device isfixed with respect to said coil and relatively movable with respect tosaid rod in the direction of the rod axis.

References Cited UNITED STATES PATENTS 2,537,289 1/1951 Van Embden219l0.79 X

3,100,250 8/1963 Herczog et a1. 219l0.79 X

3,232,716 2/1966 Quast et a1. 21910.97 X

FOREIGN PATENTS 1,358,425 3/ 1964 France.

RICHARD M. WOOD, Primary Examiner.

L. H. BENDER, Assistant Examiner.

1. IN APPARATUS FOR CRUCIBLE-FREE ZONE MELTING OF A CRYSTALLINE ROD, ANINDUCTION HEATING COIL FOR FORMING A MELTING ZONE IN THE ROD, SAID COILSURROUNDING AND SPACED FROM THE ROD AND BEING ENERGIZABLE TO PRODUCE ANINDUCTIVE HEATING FIELD AROUND THE ROD, SAID COIL AND ROD BEINGRELATIVELY MOVABLE IN THE DIRECTION OF THE ROD AXIS SO AS TO MOVE THEMELTING ZONE ALONG THE ROD, AND A CONCENTRATOR DEVICE HAVING AN OPENINGTHROUGH WHICH THE ROD EXTENDS,